This paper reports the design, proof-of-concept implementation and preliminary performance assessment of a low-cost, real-time, portable, low power, and small form factor GNSS rebroadcaster. This device can be used both as a GNSS signal generator and as a GNSS signal regenerator. This device can be used to test the addition of new features in GNSS signals, such as new signals for ranging, and to characterize the performance of new and existing spoofing countermeasures for GNSS receivers in real time. This device does not require the use of post-processed GNSS signals, enabling the testing with live signals, for instance in a vehicular test campaign replicating the correct dynamic and channel impairments.
Even though the remarkable growth of the last years, the global GNSS market will still continue to expand in the next decade both in terms of devices and service. The introduction of a new generation of mass market chips based on multi GNSS dual frequency measurements, already being commercialized and integrated in smartphones by major manufacturers, is contributing to a new level of positioning accuracy in the mass-market location-based services. Better positioning and navigation experience in urban environments are the key challenges of dual-frequency GNSS enabled receivers. Thanks to the shape of the E5 signal provided by Galileo satellites as well as the other wide-band signals available on the same band, as provided by GPS satellites, the multipath effect could be greatly reduced with significant improvement on the position accuracy. In this paper a low complexity uncombined dual frequency Extended Kalman Filter (EKF) filter is presented. The goal is to deliver continuous and smooth navigation results, even in challenging environment conditions, with a low complexity and stable solution for the GNSS mass market. The proposed EKF follows the standard and simple approach of the single frequency EKF, but exploiting the measurements from both L1 and L5 bands to take benefits of the straightness of L5 measurements against multipath. Therefore, a simple uncombined L1 and L5 measurements approach is used and it is preferred in area with not significant ionospheric error. For each epoch and for each satellite in view, only the measurements coming from one frequency are selected. To exploit the resilience of L5/E5a measurements against multipath and to overcome the visibility limitation due to lower number of satellites available on L5 band than on L1 band, a “visibility check” on the measurements available is done. To show the greater resistance of L5/E5a measurements to the effects of multipath and so to test the proposed EKF, the raw measurements of a mass market dual frequency chipset for smartphone of two kinematic data collections are used. To cover different signal reception conditions, an urban/sub-urban and a rural environment are chosen. Different PVT post processing configurations have been applied to the GNSS observations, to evaluate the impact of carrier phase pseudorange smoothing and of the different frequency selections on the position accuracy and solution availability. In addition, the performance of the proposed standard single point positioning EKF using uncombined dual frequency signals is also compared to the performance obtained by processing the same raw measurements with an external Precise Point Positioning (PPP) tool. The results are promising and show that a positioning improvement from few meters up to tens meters is possible using dual frequency mass market receivers with respect to single frequency receivers, both in mild and harsh environments.
Lunar exploration is going to have a central role in the future of space industry. A large number of missions, both scientifically and economically oriented, are planned for the next decade, many of which are consolidated and structured within international agencies, such as the ESA/SSTL Lunar Pathfinder and NASA Artemis program. This opens the door to the exploration of new technologies and techniques for missions to the moon, with the objective of improving autonomy and robustness, while reducing the costs. The use of weak-GNSS signals to navigate spacecraft in cislunar space has been investigated in several publications, bringing the advantage of being less expensive and based on already existing infrastructure, allowing to improve accuracy, robustness and autonomy. ESA is planning to demonstrate this technology as part of the Lunar Pathfinder mission. Despite the tremendous advantages brought by the use of GNSS signals in lunar orbit, this concept has some limitations, for example it cannot serve users on the far side of the Moon and the service for users experiencing Moon occultation would be compromised. Within the Moonlight initiative, ESA is currently working to define a Lunar Communication and Navigation Service (LCNS), which aims to provide advanced communication and navigation services, covering the limitations of Earth GNSS and improving further the navigation performances in cis-lunar space. This paper presents the outcome of a research aimed to perform a representative end-to-end assessment of what are realistic navigation performance achievable by an autonomous GNSS navigation system for two Moon mission scenarios. In addition, preliminary performances using a potential LCNS constellation are assessed, together with Earth GNSS or standalone. The test campaign shows the performances of two different key scenarios: Lunar Pathfinder and Low Lunar Frozen Orbit (LLFO). For the Lunar Pathfinder, it resulted that Earth-GNSS signals can be successfully utilized to reach satisfactory navigation performance: positioning errors as low as 50 meters 3D Root Mean Square (RMS) were obtained using Earth GNSS signal alone, with a slight improvement introduced by LCNS constellation. For the LLFO, the usability of Earth GNSS signal is heavily compromised by the scenario inherent characteristics. In these case, the introduction of the LCNS constellation significantly improves navigation performance, allowing to obtain results similar to the Lunar Pathfinder. This work has been performed as part of the ESA activity NAVISP-EL1-023 “Earth-Moon Navigation: System Study and development of a high-sensitive spaceborne receiver”.
The detection of evil waveforms (EWF) in GNSS signals is crucial to refrain from using anomalous signals in the PVT solution, which could degrade significantly its accuracy. The EWF detectors are, in general, based on the computation of the distortion of the code autocorrelation function. Thus, the multipath effect, an independent mechanism that also distorts the autocorrelation shape, can be incorrectly assumed by the EWF detector as the presence of EWF, leading to a major increase of the probability of false alarm. In the paper we analyze the robustness of the main EWF detectors and modulations to the presence of multipath.
Unmanned aerial vehicles have become a crucial technology in a wide variety of fields. The availability and integrity of exact positioning feedback information are of paramount importance for the safety of many UAV applications. Global navigation satellite system receivers provide reliable and accurate positioning solutions in outdoor environments. However, they suffer strong performance degradation in harsher scenarios such as forests, urban canyons, or indoor environments. This paper addresses the performance of GNSS receivers installed on a drone. They were tested in different scenarios such as under open sky or in adverse conditions of signal reception. The solutions offered by the receivers were compared to RTK reference trajectories. Moreover, the advantage of including a stereo tracking camera in the setup to obtain a more accurate reference trajectory in some scenarios was analyzed. The results give insights on the accuracy and the quality of the measurements of current GNSS technologies in aerial applications.
